fix($core): NotFound component doesn't work (close: vuejs#1130)

BTW, load Layout component synchronously

Author: ulivz

packages/@vuepress/core/lib/app/components/LayoutDistributor.vue

@@ -3,6 +3,12 @@
 </template>
 
 <script>
+import Vue from 'vue'
+import { getLayoutAsyncComponent } from '../util'
+
+Vue.component('Layout', getLayoutAsyncComponent('Layout'))
+Vue.component('NotFound', getLayoutAsyncComponent('NotFound'))
+
 export default {
  computed: {
  layout () {